Good Vibrations: Calculating Excited State Frequencies Using Ground State Self-Consistent Field Models

△SCF methods have proven to be reliable computational tools for the assignment and interpretation of photoelectron spectra of isolated molecules. These results have increased the interest in △SCF techniques for electronic excited states based on improved algorithms that prevent convergence to ground states. In this work, one of these △SCF improved algorithms is studied to demonstrate its ability to explore the molecular properties for excited states. Results from △SCF calculations for a set of representative molecules are compared with results obtained using time-dependent density functional theory and single substitution configuration interaction method. For the △SCF calculations, the efficacy of a spin-purification technique is explored to remedy some of the spin-contamination presented in some of the SCF solutions. The obtained results suggest that the proposed projection-based SCF scheme, in many cases, alleviates the spin--contamination present in the SCF single determinants, and provides a computational alternative for the efficient exploration of the vibrational properties of excited states molecules.

Tuning the Magnetic Properties of Diamagnetic Di-Blatter’s Zwitterion to Antiferro- and Ferromagnetic Diradical

<div>In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), the molecule embraced with two Blatter’s moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from zwitterionic ground state to diradicalized state. With larger coupler, remarkably strong ferromagnetic interactions are realized based on DFT and WFT based CASSCF/NEVPT2 methods. An analysis based on extent of spin contamination, CASSCF orbitals occupation numbers, HOMO-LUMO and SOMOs energy gap is demonstrated that marks the transition of ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on TPHA skeleton with diradicaloid state and in some cases, even triplet ground state.</div>

Tuning the Magnetic Properties of Diamagnetic Di-Blatter’s Zwitterion to Antiferro- and Ferromagnetic Diradical

<div>In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), the molecule embraced with two Blatter’s moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from zwitterionic ground state to diradicalized state. With larger coupler, remarkably strong ferromagnetic interactions are realized based on DFT and WFT based CASSCF/NEVPT2 methods. An analysis based on extent of spin contamination, CASSCF orbitals occupation numbers, HOMO-LUMO and SOMOs energy gap is demonstrated that marks the transition of ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on TPHA skeleton with diradicaloid state and in some cases, even triplet ground state.</div>

Effects of Polarization Function on the Spin Contamination and Distribution in β'- Me4P[Pd(dmit)2]2

The effects of polarization function on the spin contamination and distribution in β'-Me4P[Pd(dmit)2]2 were studied using the DFT cluster method. Two basis sets, SV and SVP were considered in the calculations, where B3LYP functional was employed in the doublet state of the one-fragment and dimer clusters. The values of <S2> before annihilation for both SV and SVP basis sets are excellent and very close to the perfect theoretical eigenvalue of 0.75. The values of the spin densities at thiolate and thione calculated using SVP were found to be smaller than the ones using SV. The difference, however, is less than eight percent. In contrast, the difference in the spin density at Pd atoms in both monomers is significantly larger for the SVP, being about 21%. The inclusion of polarization function resulted in the shifting of electron density from the sulfur atoms to the central Pd atoms. The calculated spin densities revealed the inhomogeneous distribution of the electron spin in the dimer that leads to the existence of electron-rich and electron-poor regions.

Extent of Spin Contamination Errors in DFT/Plane-wave Calculation of Surfaces: A Case of Au Atom Aggregation on a MgO Surface

The aggregation of Au atoms onto a Au dimer (Au2) on a MgO (001) surface was calculated by restricted (spin-un-polarized) and unrestricted (spin-polarized) density functional theory calculations with a plane-wave basis and the approximate spin projection (AP) method. The unrestricted calculations included spin contamination errors of 0.0–0.1 eV, and the errors were removed using the AP method. The potential energy curves for the aggregation reaction estimated by the restricted and unrestricted calculations were different owing to the estimation of the open-shell structure by the unrestricted calculations. These results show the importance of the open-shell structure and correction of the spin contamination error for the calculation of small-cluster-aggregations and molecule dimerization on surfaces.